A reader's Concrete and Concreting question: Efflorescence in slab?

Scott C.   from   Olivehurst, US   had this question about efflorescence in a slab.

One of the most unselfish, informative sites on building I have seen.

I have 35 yrs in building/remodel.

Regarding efflorescence: A Client calls and says small pyramids 13-18mm high appear on the hearth (floor level, slab)I am thinking that its water source is subterranean induced by poor perimeter drainage outside and introduced by hydrostatic pressure and capillary action. Any thoughts??

Bill's answer

Hi Scott, Thanks for the kind words about the site. Obviously they count a lot more coming from a guy like yourself. I have to agree with you about the efflorescence you mention.

• The only time I have seen a hint of it indoors in a finished house has been in a cellar.
• To be showing up on a ground level slab is a sign that there is no vapour barrier under that part of the slab, and to the extent that you say, well there has to be movement of water from one side to the other.
• How to fix it is another thing entirely. You are probably right about poor perimeter drainage.
• Fixing that could solve a few problems in the house, not just the efflorescence, but nothing is guaranteed if the slab itself was not laid right.
• If it is coming out of a couple of well defined cracks it might be possible to seal them with epoxy.